Automatic paper feeding machine



3 Sheets-Sheet l Y.V.Y.Y. I

J. J. THOMPSON AUTOMATIC PAPER FEEDING MACHINE Filed Nov. 15, 1938 Inventor J. J. THOMPDON Q Hnrngq5 PRINTED MATERIAL July '15, 1941.

Y 1941- J. J. THOMPSON 2,2493% AUTOMATIC PAPER FEEDING MACHINE Filed Nov. 15, 1958 s Sheets-Sheet 2 MHTERWRL.

r Inventor Httomeqs Y 15, 1941- J. J. THOMPSON AUTOMATIC PAPER FEEDING MACHINE 3 Sheets-Sheet 3 Filed Nov. 15, 1938 O'O'O'O'O'O'O'O'O IL-I'QI' MnTERmL I nven for J. J. TH oMPsoN fl ltorncqs I'll! Patented July :15, 1941 UNITED s'r' rr 'zs PATENT- OFFICE AU'roMA'rIc ran-m resume 1mm corporation of Delaware Application November 15, 1938, Serial No. 240,595

. 1 Claim. (Cl. 271-23) 4 The present invention relates to combinedpaper-feeding and paper-cutting machines and, in particular, to the art of feeding paper at a "constant rate to a cutter, thus insuring that each 'cut will be made at the same place relative to the printed matter on the paper.

When paper is being taken from a continuous roll and fed through gear-driven rolls to a knife,

the line of severance does not always occur at thesame position on the paper relative to the printed matter. Consequently, if the paper is used as wrapping material for a box containing,

for example, food stuffs, the printed material is not always positioned symmetrically on the box but actually may extend over the corners of the box, giving a bad appearance and decreased sales appeal. This change in: position between the line of severance and the printed matter may be due to many reasons, of which perhaps the most important are slippage between the paper and the feed rolls, shrinkage or expansion of the paper due to climatic conditions, change of tension in the sheet, or diil'erence in paper surface smoothness, etc., all of which may cause a w considerable variation in the rate 01' paper feed.

beam. of the photo-electric control system. This variation in the paper feed velocity is ei'iected by changing the speed of the feed rolls. Some of these arrangements have operated satisiac- T torily, but, in general, they involve great complication of apparatus includirigswitche's, control mechanism, etc., which tends to detract from their dependability of operation as well as necessitating high costoi installation. As anexample oione of these complications, it has been proposed in the photo-electric controlunit to employ a vacuum tube amplifier between thephoto electric tube and the grid controlled rectifler. Ithas also been proposed to energize the rectifier with direct current and to insert ,in the plate circuit of the rectifier a timerelay or other form of switch to open fcircultthe rectifier after correction of the feed roll speed has been attained.

is to provide an improved and simplified system of correcting the speed of the, rolls for feeding paper to-a cutter to make up for the change in length ot paper due to whatever cause.

Another object is to provide a'n= improved photo-electric control unit, particularlya system .which employs the minimum number of tubes and is also adapted to be energized in such a manner that no timing relays or other open circuiting switches need be employed in order to de-energize the rectifier tube after the feed roll speed correction has been made. In carrying out this last-named object, the invention provides a photo-electric tube whichoperates directly into a gas-filled grid-controlled rectifier and the plate circuit of the latter is. energized by alternating current.

Other objects and features will be apparent as thespecification is perused in connection with the accompanying drawings, in which- Figure 1 shows a circuit diagram of a photo electric tube system, in which the grid controlled rectifier is energized whenever a black spot intercepts the light beam which impinges on" the photo-electric tube;

, Figure 2 is a diagrammatic showing of a combined electrical and mechanical system including gearing, etc., which advantageously employs the photo-electric tube' unit illustrated in Figure 1;

' Figure 3 diagrammatically illustrates a photoelectric tube control system, in which the grid controlled rectifier is energized whenever a light spot intercepts the light which impinges on the photo-tube; Figure 4 shows a combined photo-electric tube control system and a mechanical arrangement which advantageously employs the system shown; inl 'isureih 1 Figure 5 shows still another photo-electric tube control system, in which the grid controlled rectifier is energized whenever a light spot intercepts light beam impinging onthe photo-electric tube. This circuit differs from the circuit shown in Figure 3 in severalrespects, particularly in the manner in which plate circuit of the -grid controlled rectifier is energized; and

Figure 6 illustrates a combined photo-electric tube control unit and mechanical arrangement which may advantageouslyemploy the circuit shownin-Figure 5. v v 3 Referring more particularlyto'Flgure .1, numeral I designates-the bulb of a photo-tube which containba light-"sensitive cathode 2 and with an. inert gassuch as helium or other ionizr able material, to provide sufllcient ionization :bflegrein at the operating Potentials, Atube of character is known 'onfthemarket as Iii-23. The energizing potential is preferablyto. arod-like model. The bulb ls;preierablyfli1 d3 I The primary object of the presentinvention obtained from a direct current'source 4, the negative terminal (if-which is connected to the light sensitive cathode 2. The'positive side of the battery is connected through a resistor 5 to seen that the anode of the photo-tube is connected by an equi-potential conductor directly to the grid'of the rectifier 8, and the lower end of the resistor 5 together with the positive terminal of the battery 4 are connected by an equipotential conductor to the cathode of the rectifier tube. This rectifier contains an ionizable medium and, when the grid is charged to the proper positive or less negative potential, an are or a glow discharge takes place between the anode and the cathode. A tube which has been found to operate satisfactorily. for this purpose is known on the. market as the FG-S'l.

The output circuit of the rectifier includes the plate 9 and is taken through an electromag- -net coil II to one side of a source of alternating current voltage which is impressed acrossthe primary I2 of a low frequency transformer IS. The cathode and the heater are connected togather at l4 and connected to ground at l5 and also to the other side of the alternating current mains. The heater is energized through the secondary winding I5 of the transformer 12. It will be thus seen that I employ a direct current for energizing the photo-tube and an alternating current for energizing the plate circuit of the grid controlled rectifier or Thyratron.

It will also be noted that the photo-tube works directly into the Thyratron without the inter-, position of additional amplifier between these two devices. This direct association between the photo-tube and the 'I'hyratron is made possible. by the fact that these two elements are of the gas-filled type, and their impedances sufilciently match one another efiiciently to permit an interchange of current.

The operation of the circuit is practically selfevident from the drawings. when light impinges on the light sensitive cathode 2, the'resistance, across the tube decreases and that tends to make the point A more negative which, in turn, causes a negative bias or control voltage to be applied provided in connection therewith .may be conveniently included in a scanning head located back of the feeding rolls or other convenient place. The scanning headis on anadjustable frame so that it can be moved to suit the location of the spot 25, and to and from the paper to obtain the best focus. A number of these spots 25 are provided preferably along one edge of the paper 20 and all equi-distantly spaced, as

indicated on the drawings. The spots preferably are of a black color. The paper is fed through the feed rolls 28-to a rotary knife 21. The feed rolls obtain their power through a planetary gear 28 of the type which has a stationary or reaction member 29 .and which is driven by a pinion 30. The function of the reaction member 29 will be described hereinafter. The pinion 30 is driven through a shaft indicated by the dot-dash lines II and a box of reduction gears 22 by a motor 33. The rotary knife 21 is also driven through suitable gearing indicated by the dotdash line 34 and the gearing 32 by the same motor 33. I

As pointed out hereinbefore, due certain irregularities of the paper or speed of the rolls,

it is necessary to adjust the speed of the rolls from time to time so that the knife 21 will cut the paper along a severance line determined by the position of the printing on the paper. This adjustment in feeding roll speed is brought about by the apparatus which is about to be described, and is, in turn, controlled by the photo-tube I.

-In order to efiect a change in the speed of the rolls, while they are being continuously driven by the gears 28, 30, it is necessary to rotate the reaction member 29 either in one direction or the other, depending upon whether the rolls need to be speeded up or retarded to take care of the which meshes with a worm gear 25 driven in turn by a shaft 31. This shaft is rotated through I a slotted pinion 3B driven by a gear 29:: when to the grid 6. Under these conditions, the Thyragrid 5 of the Thyratron. The latter immediately ionizes during the positive half cycle of the al-'- ternating current source, causing an extremely large current to fiow through the coil II, which immediately attracts its armaturel'l upwardly.

As shown in Figure 2, the photo-tube l is preferably housed in a box I! which is provided with 'a slit n for receiving light. The slitted box is preferably positioned 'at a different side of the paper roll 20 from the lamp 2| which is also contained in a box 22 provided-with a lens opening 23. It will be understood that the phototube, the light sourceand any optical system the slotted pinion is rigidly afilxed to' the shaft, as will be described hereinafter. normally idles on the shaft due to the turning eifect of the gear 39a until the moment'arises for the shaft to turn, as determined by the need for adjustment in the speed of the rolls.

.The mechanism by which the slotted pinion 38 is'made fast to the shaft 21, so as to cause rotation off the shaft, will now be described. Keyed to the upper end (as shown) of the shaft 31, there is a clutch 39 having a peripheral groove at the middle point and provided at the upper and lower surfaces with radially extending keys 4|, 42 respectively. The key 4| is adapted to fit into a radial slot cut into the hub '42 of a hanger 44. The lower key 42 is adapted to fit into the slot 45 of the pinion 39. The clutch 40 is adapted to slide up and down along the shaft to which it isakeyed and, when the clutch is in V its lower position so that the key 42 engages the slot 45, the clutch is adapted to rotate withthe pinion 38, thus causing the shaft 31 to rotate;

- For moving the clutch along the shaft, there is provided a bar 46 which has a bifurcated end, the furcations 41 of which straddle the clutch and loosely fit within the peripheral groove'4ll.

The bar is pivoted at 49 and is mechanically biased by a tension spring 49.

Secured to the bar, directly over the pivot This pinione a wheel I2 provided with a radially extending pin 53 which is adapted to strike the lefthand end of the bar 48' during each revolution of the shaft 54. This shaft is driven by shaftage indicated by dot-dash lines 55 connected through the gearing 32 to the motor 33. The driving gear 38a is likewise driven through shaftage 56 by the motor. The coil H is connected in the output circuit of the amplifier l as indicated in Figure 1 and the latter, in turn, is connected to the photo-tube I.

Operation of Figures 1 and 2 When the paper comes through the rolls '26 and is thereafter cut into lengths by the knife 21, the severed strips are laid across the boxes 58 which contain. foodstui'ls, etc. and which are presented by a moving belt 50 past the paper severing machine to a machine (not shown) which wraps the paper about each box. Since the latter operates automatically, it has been found on occasion that when the'wrapping paper has printed matter thereon, the printed material does not appear symmetrically and in its proper positlon on the box. This is due to the fact that the paper has not been severed at the proper place and may be caused by many reasons peculiar to the characteristics of the paper. A need, therefore, arises for adjusting this severance point or position, and this adjustment most conveniently is obtained through a. control of the speed of the feed rolls. The proper speed of the paper feed is determined according to the present invention by the use of a series of black spots 25 equidistantly spaced along one edge of the paper. Whenever a black spot intercepts thelight beam emanating from the lens 23, the internal resistance of the photo-tube I is increased, and this tends to drive the grid 6 of the Thyratron more positive and to produce ioni-zation'within the tube 3 during the positive hali cycle of the alter I to rotate due to .the driving effect of the gear light beam when the'speed oi the feed rolls is 33a. The rotating effect of the shaft 31 is communicated to the shaft 29 which, in turn, rotates the reaction member of the planetary or differential gear mechanism 28, thereby causing a slight acceleration or retardation of the speed withawhich the rolls 26 are driven by the pinion While the mechanism described up to this point is adapted to introduce either an underdrive or an over-drive oi the feed roll when the black spot intercepts the light beam, for pracerateiy set to arrive too early, and consequently the mechanism is operated to-adjust the plane-'- tary gear system until after a few adjustments, it will be found that the black spot has arrived at its properinstant of time. Under these con"- ditions, the rotary'knife 21 will sever the pape at the proper position. f

In. order that no further adjustment oi the feed roll speed will be made in case the black spot arrives at the proper time in front of the light beam, I have'provided the rotating pin mechanism 53, the rotations of which are so timed by proper gearing, etc. that the pin will strike the bar '46 at the same instant of time that the black spot should be intercepting the correct. As stated hereinbefore, every time the pin strikes the bar, the clutch 33 is disengaged from the pinion 38 so that the latter cannot rotate the shaft 31. i It is clear that the time during which vthe pin does not physically contact with the bar 46 is a time interval during which an adjustment or feed roll speed can be made bythe black spot interception of the light beam. The closer that the pin 53 comes to the bar 48, the less will be this time interval until, when the pin strikes the bar'and the black spot is in line with the light beam, no adjustment canbe made or need be made. i

From the foregoing, it is, therefore, clear that I have described a very practical-system in which a photo-electric control unit always operatesto back up the'feed, and the amountof retardation is controlled by whether the black spot arrives early or late, as determined by the instant that the pin 53strikes the bar 43. It is obvious that the position of the pin 53, in its rotating path,

can be adjusted to give a normal or average overdrive, if desired. f

It is apparent that as much oras little adjustment may be provided by this mechanism due essary in other forms oi machines, since it ist only necessary to start the paper through the rolls andstart themachine.

In Figures 3 and 4 I have shown another form a or photo-electric control unit and feed roll'speed adjustment mechanism. The control unit is so.

arranged that the Thyratron is energized when .a white spotintercepts the light beam instead of a black spot as in Figure 1. Thisunit is'constituted of a photo-tube I, preferably of thetype described in connection with Figure 1.

- photo-tube feeds into an electrostaticallycontical reasons it has been found desirable to ad- Just the mechanism including theposition or the black spot with respect to the light beam such that each adjustment of the feed roll speed is in the form of a retardation rather than acceleration. Consequently, the reaction member is rotated in only one direction. In order to cause this retardation, the positional relation between the black spot and the light beam is such that the black spot intercepts the light beam at an instant of time which is too early as determined trolled amplifier which preierably is a thyratron similar to that described in connection with Fig- The anode of the photo-tube is connected ure' 1. through a pair of batteries 33, 3|, the negative terminals or which are connected together and connectedthrough a pair .oi'parallel resistances 62, 63, to the light sensitive'cathode 2, and grid 6 of the Thyra'tron. The positive terminal of the by the periodicity of the continuously moving parts or the mechanism. Thus, the light beam is interceptedby the black spot which is delibbattery 60 is connected with the anode of the photo-tube and thepositive terminal of the battery'6l is connected to the cathode tron. v V

In the grid circuit oi. the Thyratron there is a switch 64 positioned between two separate termiof the Thyra- -nals or the resistors 32, 3, the purpose of which will be described hereinafter. The cathode of The out of engagementwith the teeth ll.

the Thyratron is heated by a filament ll preferably energized from a direct current source of 7 potential 65. The plate circuit of the thyratron includes the operating coil ll of the. electromagnet l'l, similar to Figure 1 and in addition includes a switch 8 which isconnected through the battery 01 to the cathode. A mechanism which will be described here after, opens and closes the switches II, it in synchronism; Assuming that the switches have been closed momentarily, and that a white spot i. e., the absence of a black spot) intercepts the light beam, the light sensitive cathode 2 is activated-which in effect reduces the resistance of the photo-tube. gurrent from the battery OI these combined resistances is such as to drive the grid I less negative or more positive and causes an are or glow in the tube 8 due to the ionizing effect of the potential 01. The plate current flows through the tube and through the resistors 62, 63 in parallel. The drop in potential across flowing through coil pulls the: core l'l downwardly.

However, when a white spot is passing in front of the light beam, and it so happens that the switch 04 is open at the moment, the grid circuit of the Thyratron is alsoopened, so'that the latter will not be ionized. Consequently, in order that the electro-mag'net ll, shall operate, it is necessary that the switches N and It be closed for a light spot interception or rather, to have no black spot interposed between the photo-tube and its light source.

As in the case of Figures 1' and 2, the electro- .magnet I l serves to adjust, preferably to retard the speed of the paper feed' rolls 26. The mech-' anism by which this is accomplished, now to be justment for the bar It as indicated at 82. The arm It is arranged to ride along the outside surface of a cam II which is continually rotated by-a shaft 04 connected back to the gear box 32. Consequently, as the cam 83 is rotated the bar ll reciprocates ina horizontal direction and thus gives the arm 13 an angular motion which in turn transmits a similar motion to the arm It throughthe spring Ila. Inasmuchasthearm latter is kept out of engagement with the teeth II, and the ratchet wheel ll does not turn and no adjusting effect is transmitted to the reaction member 2!. However, a mechanism which will now be descrlbed is provided for preventing the arm I. from following the arm 13 in its reciprocating movement, so that the latter will cause the pawl to engage the teeth and thereby move the ratchet wheel. This mechanism consists of a latch I] which is spring biased at it and is adapted to move on a pivot ll. The latch is secured to the core 11 of the electro-magnet ll referred to hereinbefore. It is apparent that as the coil II is energized the core I1 is pulled downwardly causing the latch 08 to engage the arm 16 and against the teeth 'II by the spring; 15.

As pointed out herebefore, the Thyratron O and coil I I become energized when alwhite spot passes described, is shown in Figure 4. Whenever an element appears in this figure corresponding to a similar element in Figure 2 it will be given the same reference character. This retardation effeet is brought about by a planetary or differential gearing 28, having -a reaction member II. The reaction member is coupled through shaftage 69 (indicated by dot-dash lines) driven by'a ratchet wheel Ill. The teeth II onthe ratchet are so arranged that the wheel turns clock-wise as indicated in Figure 4. For rotating the ratchet wheel a pawl 12 is provided which is pivoted on the end of an arm 13 carried by the shaft ll. There is a spring 15 tending tohold the pawl I2 in one of the ratchet-teeth. An arm 16, also spring 15. This is theposition shown in Figure 4.

However, when the two arms It and It are angularly displaced with respect to one another, the arm 13 which carries the pawl moves the rear surface of the latter away from the arm |I,'permitting spring 15 to force the pawl in to the teeth The arm 13 is given a reciprocating-motion through the bar 18 in any suitable manner. The

mechanism shown for this purpose consists 'of be provided to prevent a continuous energ'ization of the coil II and therefore a continuous operae 1mm beam. Since this white spot I tion ofthe adjustment mechanism as initiated ,switches are preferably of the pivoted type and are provided with rolls which bear against the- 1 surfaces of a'pair'of cams II, II respectively. The cams are rotated by a shaft ll connected to erable period of time revolution of the cam.

However, them are mounted on their common shaft in such a manner that the switbhes are closed insynchronisniionly for, a short interval of time. The input'a'nd output circuit of the tube I are thus rendered operative for only a short time interval during which the tube will tive device. Consequently, it is only duringthis test period, i. e.,.when the switches close synchronously that the coil. II can be energized and ratchet wheel II rotated in response to a need for adjustment of the speed of the paper feed an arm upright II which is'adapted to swing in a vertical plane about the pivot ll-and is mechanically biased by tension spring II. This roll by operation of the reaction member 2!. The fact that a light spot does intercept the light beam when the switches 84 and ti are closed in ter, and the paper feed necessitates adjustment.

It is apparent that when the speed of the roll is correct so that the black spot interceptsthe thiq'purposewilloccurto artiii respond to current impulses from the light sensi- 1' unison shows that the black'spot is out of regis- 4 trol unit also tending to'back up or retard the speed of the rolls until the proper speed is obtained depending on the expansion or contracting slippage of the paper.

In this-figure I have shown a reciprocating knife 9| rather than a rotary element. -This knife is actuated through a pitman 92, pivotally connected to a rotary crank 93. The latter is rotated through a shaft 94 by shaftage 95 which goes back to the gear box 32. Ihave also illustrated an optional mechanism by which the feed rolls are instantaneously stopped periodically during the time that the knife M is severing the paper. This mechanism may take the form of a bell cranklever 96 pivoted at 01 and having an upper arm: which terminates in a bifurcated portion. The furcations are contained within a a position to exposethe photo-tube, alight spot grooveon a clutch 99 which is keyed to a shaft The clutch has on its opposite faces, radially extending keys IN, the right-hand one ofwhich engages the slot in gear I02. This gear normally idles on the shaft and is rotated by pinion I03 througha shaft I00. The left-hand key IN is adapted to' engage the slot I05 in a stationary bearing member I06. The shaft I00 carries the pinion 30 which drives the differential 28, as explained hereinbefore. There is provided on the rotary crank 93 a cam I01, the outer surface of which is adapted to engage the lower'end of hell crank lever 88. It is apparent that as the member 03 is rotated by the shaft 94 in order to reciprocate the knife 9|, during the paper severing operation, the cam I01 engages the lever 06 and causes the clutch 80 to move along the shaft I00 to engage with the slot I05 and to disengage with the slot I 02. Thus no rotating effect is exerted on the shaft I00 by the gear I02 but instead, a

brake effect is produced by the meshing of the key 90 with the slot I05 and the stationary member 206. In as much as the feed rolls are thus stopped during the severing operation, the paper 'at the back of the knife does not pile up and form a loop.

Still another form of photo-control system and feed roll adiusting mechanism is shown in Fig-{ urea 5 and 6. In Figure 5, the circuit is somewhat similar to that shown and described in Figure 1 except that the photo-tube is reversed and battery 6| isconnected to the cathode thyratron.

system becomes operative when a light spot intercepts the light beam. Under these conditiens the resistances of the photo-tube will depermitting more current to flow from the battery 00 through-the tube and through the resistor 5 and tending to make the point A more positive. The increased voltage drop through 5 overcomes or diminishes the negative potential normally applied to the grid 0 by the'battery 0|, thus causing the grid to become less negative or more positive. Under these conditions an are or glow discharge is producedin the Thyratron and the coil I I is energized to attract its armature core H. The mechanism shown in Figure 6 operates'ior the most part similar to that desgibed in connectionwith Figure 4 except that a tary knife 21 is provided. The time during which a test can be made to determine whether'or not a light spot is opposite thelight beam and have the Thyratronoperate to adjust the speed control mechanism is determined by a reciprocating mask I III. This mask serves to shield the photoelectric tube from the light except at predetermined instants of time. The mask maybe given I a reciprocating angular movement in any suit-,-

able manner and as-illustrated, is arranged to engage the surface of a cam I l I which is continually rotated by a shaft H2 (indicated by dotdash lines) connected to the gear box 32; In

case the paper is out of register at'the' time-that the mask is in its elevated position,,that is, in

will intercept the light beam instead of a black spot. This will cause the Thyratront to become energized and will move the electro-magnet'l'l into the coil II,-'causing the ratchet wheel I0 to rotate and thereby move the reaction member.

20 of the planetary or diiferential gear system. On the other hand if a black spot intercepts a light beam at the time the mask exposes the photo-tube, the Thyratron will not operate, due

to the manner in which it is'connected'and no adjustment will be made. As in the case of the other figures, it desirable that an over-drive be' deliberately introduced so that the-photo-electric control unit will tend'continually to retard the speed of the paper feed rolls, until a condition is reached in which the black spots are in proper register with the light beam.

While I have shown and described difierent forms of feed roll adjustment mechanism for operating with diflerent but specific types of photo-electric control units; it is obvious'th'at other combinations may be devised, interchanging the adjustment mechanisms and the control units, depending on whether it is desired to 0D- erate the mechanism by' the registry of a black or white spot with the light beam. Each control unitand its specific adjusting mechanism offer certain advantages. For example, in the circuit shown in Figures 1' and 5 which employ a inechanical arrangement for limiting the testperiod to'an almost instantaneous period of time, an electrical arrangement is provided in the Thyratron circuit for preventing continuous ionization.

This interrupting eifect is brought about by the use of alternating current plate circuit energization which automatically stops the are at the end of each positive half cycle of current. 0n the other hand, in Figure 3, in which direct-current is employed for plate circuit energize-tion, the arc is periodically interrupted by a mechanically op-' erated switch which lends itself admirably tn the switch mechanism employed in the grid. circuit of the Thyratron. However, in. all cases, and regardless of the manner in which the time period is determined during which the position of the white or dark spot is tested, the period is of very short duration. Consequently,- extreme'accuracy can be obtained in adjusting the planetary gear system or any other controlled speed mechanism in order to'vary the feed roll speed and thereby offset the slippage, expansion, contraction or any other irregularity in the paper which might cause the severing line to respect to the printing on the'paper.

It will be understood that I desire to compre I hend within my invention such modifications as come withinthe scope of the claim and the invention.

Having thus fully described my invention, what be improperly positioned with I claim as new and desire to secure by letters Patent is:

In combination, a roll adapted to feed strip' material, a mechanical system for adjusting the registry with the light beam. said last-mentioned means comprising a rotatable crank and a pivoted. bar carrying at one end a clutch, the other end of said bar being adapted to be engaged by the'rotating crank which causes the bar to move on its pivot and causes the clutch'to disengage the adjusting mechanism from its driving shaft, and means for locking the clutch in its disengag- .ing-position until the photo-tube becomes activated by light falling on indicia out of proper registry. said last-mentioned means comprising a latch which is adapted to be released by a relay controlled by the photo-tube. I

JOSEPH J. THOMPSON. 

